1. Field of the Invention
The present invention relates to an NO.sub.x (nitrogen oxides) sensor for an exhaust gas in an automobile vehicle, and a process for producing the same.
2. Description of the Related Art
For this type of NO.sub.x sensor, the present applicant has previously proposed a semiconductor-type NO.sub.x sensor made using .beta.-type Nb.sub.2 O.sub.5 having a high sensitivity to NO.sub.x in an exhaust gas (see Japanese Patent Application Laid-open No.8-15199).
The measurement of a concentration of NO.sub.x by the NO.sub.x sensor is carried out in the follower manner: If NO.sub.x is adsorbed onto the surface of a .beta.-type Nb.sub.2 O.sub.5 layer, the NO.sub.x exhibits an electron attracting effect, thereby causing an electron which is a carrier of the .beta.-type Nb.sub.2 O.sub.5 (an n-type semiconductor) to be attracted to and restricted in NO.sub.x and hence, the electric resistance value (which will be referred to as a resistance value hereinafter) of the .beta.-type Nb.sub.2 O.sub.5 layer is increased. This resistance value is converted into a concentration of NO.sub.x concentration.
The present inventors have made various researches about the NO.sub.x sensor and as a result, they have made clear the existance of the following improving points.
(a) To allow the NO.sub.x sensor to effectively function in an exhaust gas from an automobile vehicle, it is necessary to enhance the NO.sub.x sensitivity of the NO.sub.x sensor, so that the latter has a high ratio of S/N (S represents a signal, and N represents a noise).
(b) The NO.sub.x sensor shows a relatively high sensitivity not only to NO.sub.x in an exhaust gas, but also to O.sub.2 and hence, in order to enhance the NO.sub.x sensitivity, it is necessary to reduce the O.sub.2 sensitivity.